The present invention relates to a positioner system for controlling a fluid control valve and, more particularly, to an intelligent positioner system and software routines for controlling the operation thereof.
Fluid control valves are used to control the transmission and distribution of fluids, such as liquids and gases. In such applications there often are requirements for highly reliable and accurate valves for proper performance. However, due to the physical performance of the valve, as well as the environment in which the valve operates, periodic maintenance, calibration and adjustment of the valve must be maintained throughout its operation. Furthermore, the valve can exhibit non-linear and seemingly unpredictable behavior. For example, high friction in the valve may cause stick-slip behavior when the valve is being adjusted between an open and closed position. In addition, ambient temperatures and aging affect component characteristics and cause position errors.
Many valves are fitted with actuators and positioners to control the valve movement. Furthermore, some positioners are computer controlled. However, such control has been somewhat limited. In addition, the conventional computer-controlled positioners do not adequately predict actual valve behavior. Therefore what is needed is a valve positioner that facilitates the continuous maintenance, calibration and adjustment requirements of the valve.
In response to such difficulties, a technical advance is achieved by providing a valve positioner system that includes one or more unique control methods and corresponding devices, including several routines to facilitate the continuous maintenance, calibration and adjustment requirements of the valve.
In one embodiment, a method is used for detecting limit cycling in the valve. The method first measures a plurality of positions and actuator pressures for the valve. The method then determines zero crossings for the position and pressure measurements to determine a phase relationship. The phase relationship can then be compared to a predetermined value in order to detect limit cycling.
In another embodiment, a method provides bumpless transfer from a manual mode of operation to an automatic mode of operation. The method first determines when the automatic mode has been selected. It then receives a position signal from the automatic mode for a new position and a valve position from the valve. The new position is compared to the valve position, and if it is within a predefined limit, the method allows the valve to enter automatic mode. If the new position is outside of the predefined limit, the method holds the valve in a current position, informs the user of the discrepancy in positions, and/or instructs the user to manually adjust the valve closer to the new position.
In yet another embodiment, a method calculates a unique BIAS for the positioner. The valve is allowed to move in a first direction; the BIAS value for the positioner is repeatedly calculated and averaged over a period of time, the current BIAS value being compared to the average. If the current BIAS value differs significantly from the average BIAS value, the repeated averaging is stopped, thereby stabilizing the BIAS value. Also, if the positioner tries to move the valve in a second direction, the current BIAS is replaced with the average BIAS value.
In yet another embodiment, a method is used for measuring a valve signature. First, a spool valve of the actuator is opened to increase actuator pressure. As the valve is opening, a plurality of pressure data for the actuator and position data for the valve are measured. Then the spool valve is closed to decrease the actuator pressure. As the valve is closing, a plurality of pressure data for the actuator and position data for the valve is also measured. The measured data is then analyzed to determine friction and hysteresis for the valve so that the valve signature can be determined.
In yet another embodiment, a method is used for performing diagnostics on the valve. The method first positions the valve at a fist end of a test range of valve positions. Fluid pressure to the actuator is increased until it reaches a second end of the test range. The fluid pressure to the actuator is then decreased until it reaches the first end of the test range. Throughout the movement, valve position and actuator pressure data are sampled and stored in memory for later diagnosis.
In yet another embodiment, a method provides valve response feedback to the user. A position set point for the valve is received from the valve controller and the actual valve position from the valve feedback. The actual valve position is then displayed on a graph along with a characterization of the position set point. If the valve is operating correctly, the actual valve position and the characterization of the set point will align on the graph.
In yet another embodiment, a method compensates for twist of a shaft of a rotary-type valve. First the valve positioner is calibrated while the valve is in a no-force condition. Then a gradient for the angular displacement of the rotary valve verses positioner signal is determined. A difference in actuator pressure for a predetermined valve position is determined both when the valve is in the no-force condition and when the valve is in a force condition. A correction function for the valve can then be determined from the gradient and the difference in actuator pressures.
In yet another embodiment, a method provides nonlinear position control of the valve. The method uses a firs; and second gain, one for each valve direction. The method also uses an adjustable gain to compensate for size-related and position-related valve nonlinearities. When a desired position signal for the valve is received, it can then be adjusted by the adjustable gain and either the first or second gain to determine a calculated position signal for the valve.
In yet another embodiment, a self-adjusting method is used for positioning the valve. The method first finds a current BIAS for the I/P transducer. An open-loop test on the valve is performed using the current BIAS for obtaining a system characteristic and a control parameter. The control parameter can then be tuned to achieve an improved dynamic response of the valve.
In yet another embodiment, a method is used for characterizing the valve to obtain uniformity of control performance. A first position signal is used to move the valve. At least one process variable from the valve responsive to the first position signal is measured and stored with the first position signal in the cross-reference table. A position set point can then be received, adjusted by the cross-reference table, and used as a second positioning signal to the valve.
In yet another embodiment, a method is used to position the valve in case of position feedback failure. The method stores a plurality of pressures corresponding to a plurality of positions in memory. Then, when a desired position is received and the position feedback is found to be in failure, the valve can be positioned by choosing one of the stored positions and providing the corresponding actuator pressure for positioning the valve.
In yet another embodiment, a method detects seat leakage in the valve. Whenever the valve is within a minimum controllable valve lift point for an extended period of time, a maintenance action, such as notifying the user, is performed.
In yet another embodiment, a method supports custom characterization of the valve while imposing constraints to prevent a detrimental valve characteristic. A graph including a plurality of points relating flow capacity to valve position is provided to the user. The points illustrate the valve characteristic and the user can change the characteristic by changing one or more of the points. Upon receiving a position change request from the user, the method determines if there is a detrimental valve characteristic, such as a slope reversal. If so, the user is prompted for another position change.